Understanding while...else...

Terry Reedy tjreedy at udel.edu
Thu Jan 24 00:05:39 CET 2013


On 1/22/2013 7:39 PM, Oscar Benjamin wrote:
> On 22 January 2013 23:41, Terry Reedy <tjreedy at udel.edu> wrote:
>> On 1/22/2013 3:09 PM, Ethan Furman wrote:
>>>
>>> On 01/22/2013 09:44 AM, Terry Reedy wrote:
>>>>
> [SNIP]
>>>> The else clause is executed if and when the condition is false.
>>>> Now use a real Python while statement to do the *same
>>>> thing*.
>>>>
>>>> while n > 0:
>>>>     n -= 1
>>>> else:
>>>>     n = None
>>>
>>>
>>> I understand how it works (although it did take a while for it to sink
>>> in); my gripe, and probably why it is misunderstood so often, is that
>>> nine times out of ten when I /want/ to use a while-else or for-else I
>>> only want the true/false check /once/, at the beginning of the loop.
>>
>>
>> I do not understand what you are saying. There already is only one
>> true/false check, at the beginning of the loop. If you only want the check
>> *performed* once, you would use if-else. But I presume you know this.
>
> I think he meant that he would use the else clause more often if it
> had the semantics so that the two blocks below were equivalent:
>
> # Version 1
> while condition:
>      # stuff
> else:
>      # other stuff
>
> # Version 2
> if condition:
>      while condition:
>          # stuff
> else:
>      # other stuff
>
> So he wants a convenient way to execute code only if the loop
> performed zero iterations. I think that often when people are confused
> about the else clause on while loops it is because they expect this
> behaviour (which would also be useful).

Thank you for the clarification.

If 'condition' has side effects, or if one is really bothered by 
computing it twice, Version 2 could be re-written as

if condition:
     while True:
         stuff()
         if not condition: break
else:
     other_stuff()

 > The same confusion arises with
> for loops where people expect the else clause to execute if the
> iterable was empty so that these would be equivalent:
>
> # Version 1
> for x in iterable:
>      # stuff
> else:
>      # other stuff
>
> # Version 2
> iterated = False
> for x in iterable:
>      iterated = True
>      # stuff
> if not iterated:
>      # other stuff

I see. I guess people who want version 2 will have to write it explicitly.

-- 
Terry Jan Reedy




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